fix pydantic warning

[heikoklein]

Change Summary

Pydantic gives warning messages about serializer errors and wrong type confusing the users.

Related issue number

Problem mentioned in #1066

Checklist

• [x] Start with a draft-PR
• [x] The PR title is a good summary of the changes
• [ ] PR is set to AeroTools and a tentative milestone
• [ ] Documentation reflects the changes where applicable
• [ ] Tests for the changes exist where applicable
• [ ] Tests pass locally
• [ ] Tests pass on CI
• [x] At least 1 reviewer is selected
• [x] Make PR ready to review

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.31%. Comparing base (a65f838) to head (e992544). Report is 5 commits behind head on main-dev.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main-dev #1225 +/- ## ========================================= Coverage 79.30% 79.31% ========================================= Files 131 131 Lines 20236 20234 -2 ========================================= Hits 16048 16048 + Misses 4188 4186 -2 ``` | [Flag](https://app.codecov.io/gh/metno/pyaerocom/pull/1225/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metno) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/metno/pyaerocom/pull/1225/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metno) | `79.31% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metno#carryforward-flags-in-the-pull-request-comment) to find out more.

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